The invention relates to rotary drill bits for use in drilling or coring deep holes in subsurface formations.
In particular, the invention is applicable to rotary drill bits of the kind comprising a bit body having an external surface on which are mounted a plurality of cutting elements for cutting or abrading the formation, and an inner passage for supplying drilling fluid to one or more nozzles at the external surface of the bit. The nozzles are so located at the surface of the bit body that drilling fluid emerging from the nozzles flows past the cutting elements, during drilling, so as to cool and/or clean them.
Although not essential to the present invention, the cutting elements may be in the form of so-called "preform" cutting elements in the shape of a tablet, often circular, having a superhard cutting face formed of polycrystalline diamond or other superhard material.
In one commonly used method of making rotary drill bits of the above mentioned type, the bit body is formed by a powder metallurgy process. In this process a hollow mould is first formed, for example from graphite, in the configuration of the bit body or a part thereof. The mould is packed with powdered material, such as tungsten carbide, which is then infiltrated with a metal alloy, such as a copper alloy, in a furnace so as to form a hard matrix.
Using conventional infiltration alloys, the furnace temperature required to form the matrix is usually of the order of 1000.degree. C. to 1170.degree. C. and this leads to certain disadvantages. For example, conventional polycrystalline diamond preforms are only thermally stable up to a temperature of 700.degree.-750.degree. C. For this reason the preform cutting elements, or cutting structures incorporating the elements, are normally mounted in the bit body after it has been infiltrated. The interior surface of the mould is therefore normally suitably shaped to provide surfaces to which the cutting elements may be subsequently brazed, or to provide sockets to receive studs or carriers to which the cutting elements are bonded. The subsequent mounting of the cutting elements on the body is a time-consuming and costly process, and may involve serious technical difficulties. The cutting elements and/or cutting structures must also be made sufficiently accurately to fit the pockets in the bit body, and this also adds to the cost.
There are now available certain polycrystalline diamond preforms which are thermally stable up to conventional infiltration temperatures, typically about 1100.degree. C. However, the use of such thermally stable preforms gives rise to further problems, particularly with regard to ensuring that the cutting elements are securely mounted on the bit body with sufficient exposure for optimum cutting action.
Conventionally, before the matrix is formed, the mould is partly filled with a steel blank, the matrix being formed around the blank. After the matrix forming process, a further steel piece is welded onto a projecting portion of the blank and is shaped and formed with a thread to provide the threaded shank by means of which the drill bit may be connected to the drill string. The provision of the threaded shank must be effected after the matrix has been formed since the high infiltration temperature can cause metallurgical deterioration of the steel blank.
In order to avoid the above mentioned disadvantages, it has been proposed to use a low temperature infiltration alloy such that the infiltration temperature is below about 700.degree. C., i.e. is at a temperature where conventional preforms are thermally stable. One such low temperature alloy has comprised 45% silver, 15% copper, 16% zinc and 24% cadmium. However, the use of such alloy has not proved commercially acceptable, not least because of its high cost.
The present invention therefore sets out to provide a method of making a drill bit using a low temperature infiltrant which may overcome the disadvantages of the known methods referred to above.